Dental implants have been successfully used since more than 10 years. The major part of the dental implants currently used consist of titanium, since titanium has a sufficiently low elastic modulus and also has a relatively large strength. In addition, it is of particular importance that when using titanium as an implant material a safe integral osteogenesis can be reached when the surface is suitably treated (e.g. roughened by sand blasting). This means that the titanium implants, after reaching a primary stability by screwing into the bone, safely ossify within a healing time of about 3 to 4 months so that a permanent bond between the anchoring part screwed into the bone and the bone is guaranteed. Usually two-part implants are utilized. Basically, there are to possibilities for this end:
According to a closed sub gingival system the anchoring part of the implant is embedded until the bone ridge so that the mucoperiost cover can be sewn above the implant. A drawback is the necessary secondary operation at the end of the primary healing phase for allowing a subsequent application of a mounting part and thereon the desired prosthesis or crown.
By contrast, when using the open transgingival system, then the anchoring part of the implant can be sunk in up to about 3-5 mm above the bone ridge at mucosal level, thus avoiding a secondary operation. The wound edges can be directly adapted to the implant neck portion, thereby effecting a primary soft tissue closure to the implant.
Ceramic abutments offer particular advantages during the subsequent matching of the supra-construction, such as bridges or crowns, to the abutment. They can be simply ground and allow to build constructions using prior art processes known to the dentist. Ceramic abutments offer particular advantages due to the fact that their color can be closely matched to the natural tooth color. Lately also abutments of zirconia have been developed which offer a particularly high strength.
Such a system consisting of two-part implants having an anchoring part and a mounting part, an abutment and a prosthesis applied thereon offers a good matching to the geometric situation for different indications, however, generally the multitude of the components used is detrimental for the mechanical stability of the total system. Also each further bonding leads to possible starting points for bacteria which may cause parodontitis or gingivitis with the gap.
Lately also zirconia ceramics have become available that have an extremely high strength, in particular when the shaped bodies are prepared by hot isostatic pressing or by subsequent hot isostatic densifying. Such a zirconia ceramic which may roughly comprise 92.1-93.5 wt-% ZrO2, 4.5-5.5 wt.-% Y2O3 and 3.8-2.2 wt.-% HfO2 is for instance known from U.S. Pat. No. 6,165,925.
However, the application of zirconia ceramic as a material for making the anchoring part of an implant seems not possible, since a sufficient mechanical stability of a zirconia ceramic is necessary, this requiring a highly dense preparation, practically without any porosity to be measured, this simultaneously leading to a clean cut extremely hard surface.
Such a material is bio-inert, so that no integrating osteogenesis is to be expected, this is why this material is not regarded to be suitable for the preparation of an anchoring part of an implant. In contrast, it is known that titanium covered ceramic shows extremely good results with respect to integrating osteogenesis. WO03/045268 discloses a dental implant wherein the anchoring part and the mounting part are configured in one piece from a zirconium oxide-based material. In principle titanium covered ceramic would be the material of choice. However, the formation of a strong bonding between the titanium layer and the ceramic has been a problem. This problem was in parts addressed in US 2001/0036530 A1. However, the results achieved by the methods disclosed in US 2001/0036530 A1 are not satisfactory. When the bonding strength is measured in five individual tests there are no cracks observed but the bonding strength of 67 MPa on average is only an insufficient step of over the methodology known in the prior art which will give a bonding strength of 41 MPa.
In particular there are applications of ceramic titanium combinations which require extremely high bonding strengths. Such applications are not just dental applications but also other medical applications such as bipolar hemiprostheses which are widely used for the treatment of medical fractures of the femoral neck in old patients with limited life expectancy since they allow for the surgical trauma to be reduced, and involve short rehabilitation times. The typical long-term problems encountered with such types of prostheses consist in polyethylene wear and destruction of the cartilage in the acetabulum, which as a consequence causes protrusion into the minor pelvis. In conventional dual-head prosthesis, the polyethylene used in the internal joint is subject to high mechanical strain which generally exceeds the strain rated for the material use. The accelerated wear resulting from this causes loss of sliding capacity of the internal joint, and hence prepares the way for technical failure of the conventional dual-head prosthesis. Also a ceramic dual-head prosthesis does not solve the problem. In theory, it allows for the friction inside the joint to be minimized, however, in practice such dual-ceramic prosthesis produce creaking noises and other unpleasant noises. It would therefore be advantageous to have ceramic compositions comprising titanium layers which may be used in such bipolar hemiprosthesis and which have a satisfactory bonding strength between the titanium layer and the ceramic unit thereby allowing for such applications.
The present invention solves the problems outlined above by providing for a method which leads to an extreme bonding strength between a given ceramic unit and its titanium layer.